Control of current supplied by a transistor to a pixel in an electroluminescent display device

ABSTRACT

In an active matrix electroluminescent display device, an overall brightness level of an image to be displayed in a frame period is determined. A drive transistor of each pixel is controlled in dependence on an input drive signal for the pixel and on the overall brightness level, for example using a signal processor ( 30 ) to vary the pixel drive signals. This arrangement can control the pixels to limit the maximum currents drawn by the pixels, thereby limiting the cross talk effects resulting from voltage drops along row or column conductors. If an image is bright, the pixel drive levels across the image (or at least a part of the image) can be reduced, so that the maximum brightness is reduced.

This invention relates to electroluminescent display devices,particularly active matrix display devices having thin film switchingtransistors associated with each pixel.

Matrix display devices employing electroluminescent, light-emitting,display elements are well known. The display elements may compriseorganic thin film electroluminescent elements, for example using polymermaterials, or else light emitting diodes (LEDs) using traditional III-Vsemiconductor compounds. Recent developments in organicelectroluminescent materials, particularly polymer materials, havedemonstrated their ability to be used practically for video displaydevices. These materials typically comprise one or more layers of asemiconducting conjugated polymer sandwiched between a pair ofelectrodes, one of which is transparent and the other of which is of amaterial suitable for injecting holes or electrons into the polymerlayer. The polymer material can be fabricated using a CVD process, orsimply by a spin coating technique using a solution of a solubleconjugated polymer. Ink-jet printing may also be used. Organicelectroluminescent materials exhibit diode-like I-V properties, so thatthey are capable of providing both a display function and a switchingfunction, and can therefore be used in passive type displays.Alternatively, these materials may be used for active matrix displaydevices, with each pixel comprising a display element and a switchingdevice for controlling the current through the display element.

Display devices of this type have current-addressed display elements, sothat a conventional, analogue drive scheme involves supplying acontrollable current to the display element. It is known to provide acurrent source transistor as part of the pixel configuration, with thegate voltage supplied to the current source transistor determining thecurrent through the display element. A storage capacitor holds the gatevoltage after the addressing phase.

FIG. 1 shows a known pixel circuit for an active matrix addressedelectroluminescent display device. The display device comprises a panelhaving a row and column matrix array of regularly-spaced pixels, denotedby the blocks 1 and comprising electroluminescent display elements 2together with associated switching means, located at the intersectionsbetween crossing sets of row (selection) and column (data) addressconductors 4 and 6. Only a few pixels are shown in the Figure forsimplicity. In practice there may be several hundred rows and columns ofpixels. The pixels 1 are addressed via the sets of row and columnaddress conductors by a peripheral drive circuit comprising a row,scanning, driver circuit 8 and a column, data, driver circuit 9connected to the ends of the respective sets of conductors.

The electroluminescent display element 2 comprises an organic lightemitting diode, represented here as a diode element (LED) and comprisinga pair of electrodes between which one or more active layers of organicelectroluminescent material is sandwiched. The display elements of thearray are carried together with the associated active matrix circuitryon one side of an insulating support. Either the cathodes or the anodesof the display elements are formed of transparent conductive material.The support is of transparent material such as glass and the electrodesof the display elements 2 closest to the substrate may consist of atransparent conductive material such as ITO so that light generated bythe electroluminescent layer is transmitted through these electrodes andthe support so as to be visible to a viewer at the other side of thesupport. Typically, the thickness of the organic electroluminescentmaterial layer is between 100 nm and 200 nm. Typical examples ofsuitable organic electroluminescent materials which can be used for theelements 2 are known and described in EP-A-0 717446. Conjugated polymermaterials as described in WO96/36959 can also be used.

FIG. 2 shows in simplified schematic form a known pixel and drivecircuitry arrangement for providing voltage-addressed operation. Eachpixel 1 comprises the EL display element 2 and associated drivercircuitry. The driver circuitry has an address transistor 16 which isturned on by a row address pulse on the row conductor 4. When theaddress transistor 16 is turned on, a voltage on the column conductor 6can pass to the remainder of the pixel. In particular, the addresstransistor 16 supplies the column conductor voltage to a current source20, which comprises a drive transistor 22 and a storage capacitor 24.The column voltage is provided to the gate of the drive transistor 22,and the gate is held at this voltage by the storage capacitor 24 evenafter the row address pulse has ended. The drive transistor 22 draws acurrent from the power supply line 26.

The drive transistor 22 in this circuit is implemented as a PMOS TFT, sothat the storage capacitor 24 holds the gate-source voltage fixed. Thisresults in a fixed source-drain current through the transistor, whichtherefore provides the desired current source operation of the pixel.

The above basic pixel circuit is a voltage-addressed pixel, and thereare also current-addressed pixels which sample a drive current. However,all pixel configurations require current to be supplied to each pixel.

In a conventional pixel configuration, the power supply line 26 is a rowconductor, and is typically long and narrow. The displays are typicallybackward-emitting, through the substrate carrying the active matrixcircuitry. This is the preferred arrangement because the desired cathodematerial of the EL display element is opaque, so that the emission isfrom the anode side of the EL diode, and furthermore it is not desirableto place this preferred cathode material against the active matrixcircuitry. Metal row conductors are formed, and for backward emittingdisplays they need to occupy the space between display areas, as theyare opaque. For example, in a 12.5 cm (diameter) display, which issuitable for portable products, the row conductor may be approximately11 cm long and 20 μm wide. For a typical metal sheet resistance of 0.2Ω/square, this gives a line resistance for a metal row conductor of 1.1kΩ. A bright pixel may draw around 8 μA, and the current drawn isdistributed along the row. The voltage drops can be reduced to someextent by drawing current from both ends of the row, and improvements inefficiency of the EL materials can reduce the current drawn.Nevertheless significant voltage drops are still present. This problemis worsened for larger displays, even if the total line resistance canbe kept the same. This is because there are more pixels per row, oralternatively larger pixels if the resolution is the same. The voltagevariations along the power supply line alter the gate-source voltage onthe drive transistors, and thereby affect the brightness of the display,in particular causing dimming in the center of the display (assuming therows are sourced from both ends). Furthermore, as the currents drawn bythe pixels in the row are image-dependent, it is difficult to correctthe pixel drive levels by data correction techniques, and the distortionis essentially a cross talk between pixels in different columns.

According to the invention, there is provided an active matrixelectroluminescent display device comprising an array of display pixels,each pixel comprising:

an electroluminescent (EL) display element; and

active matrix circuitry including at least one drive transistor fordriving a current through the display element,

-   -   wherein the device further comprises:        -   means for determining an overall brightness level of an            image to be displayed in a frame period; and        -   means for controlling the at least one drive transistor of            each pixel in dependence on a respective input signal            providing a drive level for the pixel and in dependence on            the overall brightness level.

This arrangement can control the pixels to limit the maximum currentsdrawn by the pixels, thereby limiting the cross talk effects describedabove. For example if an image is bright, the pixel drive levels acrossthe image (or at least a part of the image) can be reduced, so that themaximum brightness is reduced. For a dark image, the maximum allowedpixel brightness can be increased. Of course, this is a distortion ofthe image. However, it has been recognized that a similar effect can beobserved in CRT (cathode ray tube) display, where the brightness of animage is a function of the total light output. This in fact provides arealistic image. In particular, the increased brightness for smallbright areas (such as reflections of sun from water) provides arealistic appearance. The implementation of this effect in an EL displayenables the maximum current along the row conductors to be reduced, suchthat the voltage drops are not sufficient to cause noticeablenon-uniformity or cross talk in the displayed image.

In one arrangement, a signal processing device determines an overallbrightness level and processes the input signals for the pixels independence on the overall brightness level. This provides processing ofthe image data and requires no hardware modification. In this case afield store is preferably provided for storing the input signals for animage and the input signals for all pixels of the image in the fieldstore are summed to determine the overall brightness.

A look up table can be used for modifying the input signals for thestored image in dependence on the overall brightness level.

In an embodiment of the invention gamma processing is used to controlthe peak brightness of the display. The gamma parameter isconventionally used in display or image technology indicating thedisplay linearity in terms of e.g. input signal and output luminance.This may be done by recalculating or selecting a look-up table independence of the overall brightness level. As a result, for dark imagesthe maximum allowed pixel brightness can be increased to provide thesparkling effect that is known for a CRT display.

In another arrangement, digital to analogue converter circuitry is usedfor converting digital inputs into the input signal, and the digital toanalogue converter circuitry can then be controllable in dependence onthe overall brightness level. In this case, the pixel drive signals areagain modified before application to the pixels, but at the D/Aconversion stage.

In other arrangements, the pixel configuration is used to provide theimage modification.

In a first example, the active matrix circuitry can comprise first andsecond drive transistors in parallel each connected between a respectivepower supply line and the EL display element. The first drive transistoris supplied with a first supply voltage and the second drive transistoris supplied with a second supply voltage, with at least one of thesupply voltages being variable in dependence on the on the overallbrightness level. This enables the combined current supplied by the twodrive transistors to be varied by setting the voltage of one supplyvoltage. This pixel arrangement is a modification of a conventionalvoltage addressed pixel.

The first supply voltage may be fixed and the second supply voltagevariable, and the range of variation can include the first and secondsupply voltages being equal.

In a second example with a current driven pixel, the active matrixcircuitry comprises current sampling circuitry for sampling an inputdrive current, the current sampling circuitry having a current samplingtransistor and a drive transistor in parallel each connected to arespective power supply line. Each of the current sampling transistorand the drive transistor can supply current to the display element, andat least one of the supply voltages of the power supply lines isvariable in dependence on the overall brightness level. This pixelarrangement is a modification of a conventional current addressed pixel.

The invention also provides a method of addressing an active matrixelectroluminescent display device comprising an array of display pixels,in which each pixel comprises an electroluminescent (EL) display elementand active matrix circuitry including at least one drive transistor fordriving a current through the display element, the method comprising:

determining an overall brightness level of an image to be displayed in aframe period; and

controlling the at least one drive transistor of each pixel independence on a respective input signal providing a drive level for thepixel and in dependence on the overall brightness level.

The overall brightness may be a measure of the total drive level for allpixels or an average value, and this depends on the specificimplementation. This method enables the total current to be kept withinlimits by reducing the maximum brightness for generally bright images.

Controlling the at least one drive transistor may comprises processingthe input signals for the pixels in dependence on the overall brightnesslevel and then applying the processed input signals to the pixels. Forexample, the input signals may be modified using a look up table, theaddress of which is selected in dependence on the input signal and theoverall brightness level.

If the input signals are in digital form, controlling the at least onedrive transistor can comprise controlling the digital to analogueconversion of the digital input signal in dependence on the overallbrightness level and then applying the analogue input signals to thepixels.

If the input signal comprises a current, controlling the at least onedrive transistor may comprise sampling the input current using asampling transistor, and supplying the display element with current fromthe sampling transistor and a drive transistor in parallel, wherein thesupply voltage to at least one of the sampling transistor and the drivetransistor is varied in dependence on the on the overall brightnesslevel to vary the total current supplied to the display element.

The invention will now be described by way of example with reference tothe accompanying drawings, in which:

FIG. 1 shows a known EL display device;

FIG. 2 is a simplified schematic diagram of a known pixel circuit forcurrent-addressing the EL display pixel using an input drive voltage;

FIG. 3 shows a simplified schematic diagram of a first example ofdisplay device of the invention;

FIG. 4 shows in greater detail the implementation of FIG. 3;

FIGS. 5A to 5C show some possible drive schemes which can be implementedwith the circuit of FIG. 4;

FIG. 6 shows a simplified schematic diagram of a second example of howto modify a display device in accordance with the invention;

FIG. 7 shows a first example of a modified pixel for a display device ofthe invention;

FIG. 8 shows possible drive schemes which can be implemented with thepixel circuit of FIG. 7; and

FIG. 9 shows a second example of a modified pixel for a display deviceof the invention.

It should be noted that these figures are diagrammatic and not drawn toscale. Relative dimensions and proportions of parts of these figureshave been shown exaggerated or reduced in size, for the sake of clarityand convenience in the drawings.

The invention provides an active matrix electroluminescent displaydevice in which an overall brightness level of an image to be displayedis determined, and the maximum pixel drive current within the fieldperiod corresponding to that image is controlled in dependence on theoverall brightness level. In particular, the pixel drive levels for allpixels can be scaled in dependence on the overall brightness.

Limiting the maximum currents drawn by the pixels reduces cross talk.The resulting distortion of the image has been found to improve realismrather than detract from it.

FIG. 3 shows a first way of implementing the invention. The pixel drivesignals are provided to signal processor 30 which modifies them independence on the combined (integrated) brightness of all pixels in theimage. The modified drive signals 32 are used to drive the display 34 inconventional manner. The processor adjusts the pixel drive signals(which may be currents or voltages) so that the peak pixel current andtherefore brightness is higher for images where only a small part isvery bright that for images where a large part is bright. This providesprocessing of the image data and requires no hardware modification.

FIG. 4 shows one possible implementation of FIG. 3 in greater detail. Afield store 36 is provided for storing the input signals for a completeimage, and the input signals for all pixels of the image are summed atthe same time in a summing unit 38 to determine the overall brightnessof the image. The summing unit thus outputs the combined pixel drivesignals for the image stored in the field store 36.

A look up table (LUT) 40 is used for modifying the stored image pixeldrive levels drive in dependence on the overall brightness level at theoutput of the summing unit 38. In particular, a signal 42 proportionalto the sum of the brightness values of the incoming signal over a fullfield period is passed to a look up table address generator 44, whichgenerates an address of the look up table to which the pixel drivelevels of the stored image are applied before being used to drive thedisplay. The look up table 40 essentially comprises two or more tableswhich are provided with different characteristics, and the selection ofwhich table is used to convert the data is dependent on the brightnessinput. The field store requires a one frame delay to be implemented.

By processing the pixel drive signals, many different drive schemes canbe implemented, either in hardware (with look up tables for example) orin software.

FIG. 5 shows three possible drive schemes. In each of FIGS. 5A to 5C,the graphs show how an input pixel drive level drive is modified toprovide the output. The input and output may simply be considered as theoriginal brightness level and the modified brightness level.

In FIG. 5A the three characteristics 1 to 3 are different linear gainvalues. Plot 1 provides no modification and is used for low brightnessimages where the maximum brightness can be tolerated. Plots 2 and 3decrease the pixel brightness by different ratios, for images which arebright over progressively larger areas.

In FIG. 5B, plots 2 and 3 are non-linear and in FIG. 5C all three plotsare non-linear. In each case, plot 1 is for the lowest brightness imageand plot 3 is used for the highest brightness image.

The characteristic of FIG. 5C can be used for gamma processing in orderto obtain the sparkling effect. This gamma correction is necessarybecause in current TV systems, the input video signals are processed tobe displayed on a CRT display. On such a CRT display, the relationbetween the input signal and the output luminance L is of the formL=(input data)^(γ), with γ between 2 and 3, resulting in a non-linearshape as in FIG. 5C. If the employed display has a different relation,the input data should be corrected accordingly, which is usually done bymeans of a Look-Up-Table. This correction mechanism may be adapted tocontrol the maximum brightness of the display pixels via the diagramshown in FIG. 4. The video data is stored in memory (36). The overallbrightness level of the image is determined (38) and the gammacorrection LUT (40) is altered by a LUT generator (44) to set a certainmaximum brightness depending on the overall brightness level. The totalrelation between input data and displayed luminance should have theshape of FIG. 5C. Images with a low overall brightness level will have ahigher maximum output (curve 1) value than images with a high overallbrightness level (curve 2 or 3).

FIG. 5 shows three possible scaling values for the image, but of coursethere may be many more, to a limit where is a continuous change in drivecharacteristics with brightness level.

In FIG. 4, the image modification is performed with look up tables. Ofcourse, the modification of the pixel drive signals may be under thecontrol of an algorithm or other software implementation. For example,the linear case of FIG. 5A can be implemented simply with a multiplierwith a gain control signal (i.e. a control input for the multiplier)being derived from the overall brightness.

In FIG. 4, the analogue drive signals are modified before being used todrive the display. The image data will typically originally be indigital form, and in this case it can be manipulated in software muchmore readily.

Another alternative is shown in FIG. 6, in which the digital to analogueconverter circuitry used for converting the digital image data into theanalogue drive signals inputs is modified. The control voltages 50 forthe D/A converter 52 are generated by voltage supply circuitry 54. Forexample, the D/A converter can be a resistor chain, and the inputvoltages which define the voltages on the resistor chain can be switched(schematically shown at 56) to very the output range and the way theoutput voltage varies across the range of digital input words. Thecontrol 56 is then dependent on the overall brightness of the image.Again, the pixel drive signals are modified before application to thepixels, but at the D/A conversion stage.

The manipulation of the image data provides the flexibility to implementnumerous addition functions. These may optimize the system forparticular display types or for particular types of image.

A timing controller can be incorporated which prevents sudden changes ingain from one field to the next. If small steps in gain are implemented,then when a change in overall brightness is detected, it may bedesirable to step slowly from the current look up table (or algorithm,or D/A control) to the desired one in stages, so that sudden changes inthe image are avoided. The same rate of change may be applied forincreases in gain as for decreases in gain, or they may be different.

The overall brightness may take account more of the certain parts of theimage, for example the center of the image. This may be appropriate ifconnections to the row and column conductors are made all around thedisplay, because the resistance to the edges is much lower for pixelsnear the display edge so that the currents drawn by these pixels haveless effect on the cross talk problem. The “overall brightness” thus maybe derived from a portion of the image in the center or else maycomprise a weighted measure with parts of the image near the edgecontributing less to the summation.

In the examples above, the image data is modified before being appliedto a conventional display device in conventional manner. It is alsopossible for the pixel configuration to be modified to provide the imagemodification.

FIG. 7 shows an arrangement in which the voltage driven pixelarrangement of FIG. 2 is modified to provide control of the peakbrightness in accordance with the invention. All of the circuit elementin FIG. 2 are repeated in FIG. 7 with the same reference numbers. FIG. 8shows the transfer characteristic of the circuit.

The circuit is modified by providing a second drive transistor 60 inparallel with the first drive transistor 22, and connected between itsown respective second power supply line 62 and the EL display element 2.The first and second drive transistors can thus be supplied withdifferent supply voltages. The power supply line 26 has a fixed voltageV1 applied to it, but the voltage V2 applied to the second power line 62can be varied in dependence on the image content.

If the overall image brightness is low, then the supply voltages aremade equal, V1=V2, and the transfer characteristic is steep (see the topplot in FIG. 8) because the two drive transistors are in parallel. Ifthe overall brightness increases to a point where problems with excessvoltage drops occur in the conductors, then the voltage V2 is reduced toreduce the gate-source voltage. This means that the second drivetransistor 60 is turned off at low values of input drive level (i.e. lowgate-source voltages), and depending on the exact value of V2, thesecond drive transistor 60 starts to turn on for higher brightnesslevel, but still operates at a lower current than when V1=V2. Thus, thetransfer characteristic in FIG. 8 is less steep and the peak brightnessis lower, hence the peak currents flowing.

In this arrangement, the combined current supplied by the two drivetransistors is varied by setting the voltage of one supply voltage.

The circuits of FIGS. 2 and 7 are only example of voltage driven pixels,and other possibilities will be apparent to those skilled in the art.

FIG. 9 shows a current driven pixel layout modified in accordance withthe invention.

The pixel 1 has current sampling circuitry for sampling an input drivecurrent on the column conductor 6. The current sampling circuitry has acurrent sampling transistor 70 and a drive transistor 72 in parallel,each connected to a respective power supply line 74, 76. The currentsampling transistor 70 and the drive transistor 72 can supply current tothe display element 2.

The current to be sampled is supplied to the pixel through an addresstransistor 16, and a storage capacitor 24 stores a gate source voltageof the drive transistor 72, as in the pixel arrangement of FIG. 2.

To address the pixel circuit of FIG. 9, the voltages on the two powersupply lines are equal, namely V1=V2. The address transistor 16 isturned on, and a first isolating switch 78 isolates the input currentfrom the display element. A second isolating switch 80 is closed toallow charge to flow to the storage capacitor. When the circuit hasreached a stable state, the current drawn by the column conductor 6 issourced by the sampling transistor 70, and the storage capacitor holdsthe corresponding gate-source voltage of the sampling transistor. If thetwo transistors 70, 72 are matched, this also corresponds to thegate-source voltage of the drive transistor 72 for the same current.

The current mirror can however be asymmetric with the two transistorshaving different sizes—in this case the pixel itself provides some gain.

All pixels are programmed (i.e. the storage capacitors charged) withV1=V2. Furthermore, the cathode of the EL display element 2 is held highby switch 82 to reverse bias all the display elements. Once the averageor combined brightness is known, the power level V2 is reset accordingto overall brightness.

If the overall brightness is low, then power level V2 is set just belowV1 so that bright pixels (at least) receive current from both thesampling transistor and the drive transistor. If the overall brightnessis high, then power level V2 is set lower to completely turn off thesampling transistor.

After the value of V2 is set, the switch 82 switches to earth to turn onthe display elements and the isolating switch 78 is closed and switch 70open, so that both transistors can supply current to the display element2.

The pixel transfer characteristic is again modified by selection of V2,and the current mirror pixel has the advantage that non-uniformity oftransistor characteristics is no longer an issue (as it is with thecircuit of FIG. 2). A field store is not required in this case. Instead,an accumulator can sum the drive currents during the programming stageto enable the overall brightness to be evaluated. Thus, the field periodis divided into two parts—a pixel programming part when the LEDs are offand an LED driving part where no pixels are programmed. The pixels thusact as the field store. Whilst the pixels are being programmed, hardwarein the driver circuitry will be accumulating the data to find a totalbrightness figure by the time all pixels have been programmed. Thisallows the level of the second power line to be set and then the LEDsare driven.

The isolating switches are of course implemented as transistors.

Essentially, the invention involves determining an overall brightnesslevel of an image to be displayed in a frame period; and controllingeach pixel in dependence on the original pixel drive signal and independence on the overall brightness level. As will be apparent from theabove, there are numerous ways in which this can be implemented, eitherin hardware or in software and either in the digital or analogue domain.The invention can be used for voltage or current addressing schemes.

Various modifications will be apparent to those skilled in the art. Forexample, the circuits above use PMOS drive transistors. There are alsoNMOS implementations.

1. An active matrix electroluminescent display device including an arrayof display pixels, comprising: an electroluminescent (EL) displayelement; active matrix circuitry including first and second drivetransistors for driving a current through the display element, whereinthe first and second drive transistors are in parallel, each connectedbetween a respective power supply line and the EL display element, theinput to the pixel being provided to the gates of the first and seconddrive transistors, and wherein the first drive transistor is suppliedwith a first supply voltage and the second drive transistor is suppliedwith a second supply voltage, at least one of the supply voltages beingvariable in dependence on the combined brightness level; means fordetermining a combined brightness level of a multitude of pixels in animage to be displayed in a frame period; and means for controlling thefirst and second drive transistors of each pixel individually independence on a respective input signal providing a drive level for thepixel and in dependence on the combined brightness level of themultitude of pixels in the image, wherein the means for controlling thefirst and second drive transistors comprises a signal processing devicefor determining an combined brightness level and for processing theinput signals for the pixels in dependence on the combined brightnesslevel, wherein the signal processing device is adapted to employ gammacharacteristics for processing the input signals in dependence on thecombined brightness level, wherein said gamma characteristics comprisinga gamma correction LUT altered by a LUT generator to set a certainmaximum brightness level depending on the combined brightness level. 2.The device as claimed in claim 1, wherein the signal processing devicecomprises a field store for storing the input signals for an image and asummation unit for summing the input signals for the multitude of pixelsof the image in the field store to determine the combined brightness. 3.The device as claimed in claim 2, wherein the signal processing devicefurther comprises a look up table for modifying the input signals forthe stored image in dependence on the combined brightness level.
 4. Thedevice as claimed in claim 3, wherein the signal processing device isadapted to calculate or select the look-up table in dependence on thecombined brightness level.
 5. The device as claimed in claim 1, whereinthe signal processing device operates to reduce the maximum brightnesslevel to which any pixel is driven in response to an increase in thecombined brightness of an image.
 6. The device as claimed in claim 1,wherein the signal processing device comprises digital to analogueconverter circuitry for converting digital inputs into the input signal,and wherein the digital to analogue converter circuitry is controllablein dependence on the combined brightness level.
 7. The device as claimedin claim 1, wherein the input to the pixel is provided to the gates ofthe first and second drive transistors through an address transistor. 8.The device as claimed in claim 1, wherein the first supply voltage isfixed and the second supply voltage is variable.
 9. The device asclaimed in claim 8, wherein the first and second supply voltages can beequal.
 10. A method of addressing an active matrix electroluminescentdisplay device comprising an array of display pixels, anelectroluminescent (EL) display element and active matrix circuitryincluding first and second drive transistors for driving a currentthrough the display element, the method comprising: determining acombined brightness level of a multitude of pixels in an image to bedisplayed in a frame period; wherein the means for controlling the firstand second drive transistors at least one comprises a signal processingdevice for determining an combined brightness level and for processingthe input signals for the pixels in dependence on the combinedbrightness level, wherein the signal processing device is adapted toemploy gamma characteristics for processing the input signals independence on the combined brightness level, wherein said gammacharacteristics comprising a gamma correction LUT altered by a LUTgenerator to set a certain maximum brightness level depending on thecombined brightness level, and controlling the first and second drivetransistors of each pixel individually in dependence on a respectiveinput signal providing a drive level for the pixel and in dependence onthe combined brightness level of the multitude of pixels in the image,and wherein the first and second drive transistors are in parallel, eachconnected between a respective power supply line and the EL displayelement, the input to the pixel being provided to the gates of the firstand second drive transistors, and wherein the first drive transistor issupplied with a first supply voltage and the second drive transistor issupplied with a second supply voltage, at least one of the supplyvoltages being variable in dependence on the combined brightness level.11. The method as claimed in claim 10, wherein controlling the first andsecond drive transistors comprises processing the input signals for thepixels in dependence on the combined brightness level and then applyingthe processed input signals to the pixels.
 12. The method as claimed inclaim 11, wherein determining the combined brightness level comprisesstoring the input signals for an image and summing them.
 13. The methodas claimed in claim 11, wherein processing the input signals comprisingmodifying the input signals using a look up table, the address of whichis selected in dependence on the input signal and the combinedbrightness level.
 14. The method as claimed in claim 11, whereinprocessing of the input signals is performed by employing gammacharacteristics of the array of display elements.
 15. The method asclaimed in claim 10, wherein the control of the first and second drivetransistors reduces the maximum brightness level to which any pixel isdrive in response to an increase in the combined brightness of an image.16. The method as claimed in claim 10, wherein the input signals are indigital form, and controlling the first and second drive transistorscomprises controlling the digital to analogue conversion of the digitalinput signal in dependence on the combined brightness level and thenapplying the analogue input signals to the pixels.